Mechanics L6 PPQs Flashcards
An experiment involves measuring the time taken for a ball to fall through different distances. The maximum distance is 1.5 m. One student says that it is better to use light gates or sensors and a data logger. Another student says that just using a stopwatch better. Discuss the advantages and disadvantages of each method. [4]
- light gates:
- Power needed – disadvantage
- No need to account for reaction time
- Reading to the nearest millisecond
Stopwatch:
- Simple to operate/set up
- Graph would have to be drawn manually
- Cheaper, easily possible
The student is asked to determine a value for the acceleration of freefall (g) by timing, a falling steel ball. (Core practical with electromagnet and switch)
Explain what date will be collected and how it will be used to determine g [5]
- Times for at least five Heights
- Graph should be a straight line
- Determine gradient
- Use s=1/2 at^2
- Multiply gradient by two
The student is asked to determine a value for the acceleration of freefall (g) by timing, a falling steel ball. (Core practical with electromagnet and switch)
Identify the main sources of uncertainty and/or systematic error [2]
- time taken by electromagnet to release ball
- Parallax in specified measurement
- 0 error on height or timer
The student is asked to determine a value for the acceleration of freefall (g) by timing, a falling steel ball. (Core practical with electromagnet and switch)
Comment on safety [1]
- dont tread on ball
- wear shoes
- no major hazards bc low voltage
To determine g, the student uses the equation v^2=u^2+2as
Explain why a graph of v^2 on the Y axis and s on the x axis should be a straight line through the origin [3]
- a = constant, a=g
- v^2=2as —> y=mx
- v^2 is directionally proportional to s provided a=constant and u=0
State one assumption that must be made when calculating the work done against air resistance [1]
- frictional forces are negligible
-force of gravity + air resistance are the only significant forces acting to oppose the motion
In a bungee jump, the jumper falls from a high platform while attached to an elastic cord. The cord is also attached to the platform. The cord slows the bungee jumper down, so that he comes to rest before reaching the ground.
The fall can be divided into three stages:
1 - the jumper is in free fall until the cord starts to stretch
2 - the cord is stretching until the acceleration of the jumper decreases to zero
3 - the cord continues to stretch until the jumper is momentarily at rest
Explain, in terms of work done, how the kinetic energy of the bungee jumper changes during the three stages of the fall [6]
1: KE increases of the jumper
Because work is done by gravitational force
2: KE of jumper is increasing but at a decreasing rate
Bc work is done on the cord (as it stretches)
3: KE of jumper is decreasing
Bc work is done on the cord at a greater rate than the gravitational force does work on the jumper
OR he comes to rest bc the total work done by gravitational forces is equal to work done stretching the bungee.
Equivalent to 1 kilowatt-hour
A 0.28 J
B 0.28 W
C 3.6 x 10^6 J
D 3.6 x 10^6 W
C
1x10^3 x 60 x 60
What quantity has the SI base unit kgm2s-3
Power
A water pump causes 200g of water to be ejected from the nozzle of a garden house each second at a velocity of 3m/s
Find an expression that could be used to determine the minimum output power inwatts required from the pump
0.2 x 3^2 /2
P=wd/t
KE = wd
1/2mv^2
‘If you hold an apple in your hand it’s about a newton, if you raise it through one metre, that;s about a joule and if you do it in one second, that’s about a watt’
Assuming that the apple has a mass of 100g, explain and justify the statements made about newton. Joule and watt
[6]
- w=mg
- w = o.1 x 9.81 = 1N
- GPE=mgh , 0.1 x 9.81 x 1 = 0.98J
- P=wd/t
- P= 0.98/1 = 0.98W
A student is investigating projectiles. He fires two small identical balls, A and B, simultaneously.
(A is thrown from a height above the x axis with a horizontal velocity. B is thrown from the floor with both vertical and horizontal components.)
The balls land at the same instant at the target , T
The paths AT and BT have different lengths. However, balls A and B take the same time to reach target T. Explain how this is possible. [4/5]
- Average velocity of A is greater than B.
- Bc A continually accelerates whereas B slows down
- Va = horizontal component of Vb because they travel the same horizontal distance
- vertical component of projectile motion does not affect the horizontal component
When asked to run one complete lap around a track, a student says, ‘however fast I run, my average velocity for the lap will be zero’
Comment on his statement [3]
- correct because his displacement is zero because he finishes where he started
- velocity is a vector quantity
- velocity = displacement/ time
A car is driven around a bend at a constant speed.
Explain what happens to its velocity [2]
- direction is changing
- so velocity is changing
A coin is flicked off a table so that it initially leaves the table travelling in a horizontal direction with a speed of 1.5 m/s.
A coin of greater mass is flicked with the same horizontal speed. Compare the paths of the coins. Explain your answer
[3/4]
- follows same path
- all objects have the same acceleration due to gravity
- horizontal motion is unaffected by any force ( it is independent of vertical component)
Martha throws two stones with the same force, one stone has a mass of 30g the other 60g. Explain which stone will have the greater initial acceleration.
[2]
60g will have a lower initial acceleration
Because a = inversely proportional to mass for a fixed resultant force
A student is travelling forwards at a constant speed on a skateboard. The student jumps forward off the skateboard
Explain how newtons laws of motion apply to the student and the skateboard as he jumps forward off the moving skateboard [6]
- student applies a force to the skateboard
- by N3L, the skateboard also applies a force to the student
- the two forces are equal in size and opposite in direction
- according to N2L, the student accelerates
- the skateboard decelerates according to N2L
- the skateboard has a higher magnitude of acceleration than the student as it has a smaller mass.
Two small identical solid metal spheres, A and B, are suspended by light inextensible threads from a frame.
Sphere A is pulled to one side and released. Sphere A collides with sphere B and stops and sphere B swings upwards.
Using newtons laws of motion, explain the motion of the spheres during the collision in terms of the forces acting on them. [6]
- sphere A applies a force to sphere B
- by N3L, sphere B applies a force to sphere A
- the two forces are equal in size and opposite in direction
- according to N2L, sphere A decelerates
- the force opposes the motion of sphere A, therefore it decelerates
- according to N2L, sphere B accelerates
- the magnitude of acceleration is equal because the spheres are identical meaning they have the same mass.
When stout is poured, it contains many spherical bubbles of gas which rise and form the foamy ‘head’ at the top of the drink. The manufacturers of the drink state ‘it takes 120 seconds for the head to form’.
A bubble, initially at rest in the liquid, accelerated until it achieves a uniform upward velocity.
Explain the motion of the bubble [4]
- initially drag is zero + upthrust is greater than weight
- unbalanced forces so accelerates upwards
- as velocity increases, drag increases
- until drag + weight = upthrust
- so RF = 0, a=0 , v= constant
A trebuchet is a medieval catapult designed to project a rock over large distances. The rock is held in a sling. When the peg is removed the counter weight falls and the rock is projected through the air. Student assumes that all the gravitational potential energy transferred from the counter weight is transferred to the kinetic energy of the rock.
Give three reasons why this is not correct.
- work is done against friction and energy is transferred to the surroundings.
- some kinetic energy is going into the beam/sling/counterweight.
- beam/ sling gain GPE
A trebuchet is a medieval catapult designed to project a rock over large distances. The rock is held in a sling. When the peg is removed the counter weight falls and the rock is projected through the air
The mass of the counter weight was reduced. The trebuchet was then used to project the rock again. Explain why the horizontal distance travelled by the rock decreases.
- counterweight transfers less GPE
- so transfer of Ek is reduced
- therefore rock has smaller vertical component of velocity
- time of flight will be reduced
- horizontal component of velocity is smaller.
A ball is dropped and bounces
Using the law of conservation of energy, explain the pattern of the ball’s path.
- high GPE
- ball falls, accelerates under gravity, increasing magnitude of velocity and Ek.
- on floor, GPE = 0
- bounces: velocity decelerates, Ek decreases, GPE increases
- energy is being transferred between GPE and Ek
- max height decreases with each bounce
- energy cannot be created or destroyed
- energy is transferred as thermal energy.
(Velocity time graph to show a car that accelerates and then decelerates (shape of graph is like a mountain))
Discuss, with reference to the graph, the factors that would be used to predict the motion of the car over the 120s
[6]
- at higher speeds the force of air resistance is greater.
- this will cause the resultant force and therefore the acceleration to decrease.
- ignition of the rocket engine will cause the resultant force to increase.
- as fuel is used up, mass decreases and therefore acceleration increases.
- when brakes are applied, resultant force will be backwards and vehicle will decelerate.
Explain why the assumptions about the acceleration in a SUVAT equation may not be correct in practice [2]
- air resistance increases with speed so acceleration decreases (at higher speeds)
- car could brake with greater negative acceleration than positive acceleration
